A fuel cell system is configured to supply a fuel gas and an oxidizing agent gas to a fuel cell via corresponding gas supply passages to generate electricity through electrochemical reaction of these gases within a fuel cell body. Water is generated in the fuel cell body by the electrochemical reaction, and this water is contained in an oxidizing agent off-gas (an oxidizing agent gas discharged from the fuel cell body) that is discharged from the fuel cell body. It is noted that water is also normally contained in a fuel off-gas (a fuel gas discharged from the fuel cell body) since fuel gas is supplied to the fuel cell body via an electrolyte.
Therefore, if the outside air temperature falls to the freezing point or lower when the fuel cell system is at a standstill, the moisture contained in the gases remaining in the valves, pipes, and other component parts arranged in the gas passages of the system may condense and freeze. In such a case, it may not be possible to activate the fuel cell system even when appropriate measures are taken to activate the fuel cell system; or even when the system is activated, the system may not operate properly, for example.
In view of such a problem, a method is proposed for removing residual moisture by scavenging the oxidizing agent gas passage and the fuel gas passage of the fuel cell system using an oxidizing agent gas (air) supplied from a compressor prior to halting the fuel cell system (See Japanese Laid-Open Patent Application No. 2002-313395).